The present invention relates to a plasma display device, and, more particularly, the invention relates to a plasma display device which is provided with means for enabling an image to be displayed with variable density in a predetermined number of display gradations on a screen, while enabling brightness control for the entire image without restricting the predetermined number of display gradations. The invention relates to a plasma display device for displaying an image on a screen by controlling the brightness and tone of the image, by means of, for example, a time sharing drive method, and by selectively illuminating pixels arranged in a matrix shape, and to a method of driving the device.
To provide an explanation of conventional brightness control in a matrix type plasma display device provided with means for enabling brightness control, a description will be made of the plasma display device shown in FIG. 2.
FIG. 2 is a block diagram of a plasma display panel (PDP) having a structure of the so-called "AC type". The plasma display device is composed of an analogue input circuit 10 to which an analogue video signal is inputted, an A/D converter 11, a data writing processing circuit 12, a frame memory 13, a data reading processing circuit 19, a display control circuit 15, a brightness control circuit 16, a plasma display panel 21, address electrodes 26, scanning electrodes 27 and sustaining electrodes 28, an address pulse output circuit 22 for driving the address electrodes 26, a scanning pulse output circuit 23 (used for both scanning and sustaining, but hereinafter, referred to as scanning pulse output circuit) for driving scanning electrodes 27, and a sustaining purse output circuit 25 for driving the sustaining electrodes 28.
An analogue video signal received at the input circuit 10 is converted into digital data by the A/D converter 11, and thereafter this data is written in the frame memory 13 through the data writing processing circuit 12. The data read out from the frame memory 13 is inputted to the address pulse output circuit 22 through the data reading processing circuit 14. The data which is converted into a plurality of bits by the A/D converter 11 is stored and processed in parallel when written in the frame memory 13, and the data is re-ordered in a single bit stream, in units of so-called bit frames for processing, when the data is read out from the frame memory 13. Each bit is allocated to a respective sub-field in accordance with a brightness weighting factor.
A pulse signal supplied to the address pulse output circuit 22, the scanning pulse output circuit 23 and the sustaining pulse output circuit 25 is produced by the display control circuit 15 on the basis of a vertical synchronizing signal. The brightness for the entire screen is controlled by controlling the analogue input circuit 10 using the brightness control circuit 16.
The plasma display panel 21 has two sheets of glass plates, address electrodes 26, scanning electrodes 27, sustaining electrodes 28, barrier ribs for partitioning the space between the glass plates, and the like. A pixel consists of a discharge cell which is formed in the space between the two streets of grass plates and is partitioned by barrier ribs.
The AC type plasma display panel is characterized in that the scanning electrodes 27 and the sustaining electrodes 28 are covered with dielectric layers. The discharge cell is charged with a rare gas, such as, for example, He--Xe and Ne--Xe, and when a voltage is applied between any pair of the address electrodes 26, scanning electrodes 27 and sustaining electrodes 28, a discharge occurs, generating ultraviolet rays. The barrier ribs are coated with a phosphor and are excised by ultraviolet rays to emit light. A color display can be generated by providing cells with luminous colors of phosphor, i.e. red, green or blue, for each discharge cell as a coating, to be selected in accordance with the image signal.
FIG. 3 shows an AC type plasma display drive waveform diagram. The electrode is driven in line sequence, and address pulses 51 at voltage VA are sequentially transmitted to address electrodes 26 corresponding to the discharge cells of the Nth row in response to the image signal. On the other hand, scanning pulses 52 at voltage VS are transmitted to the scanning electrodes 27 sequentially from the 1st line. In a cell for which the address voltage VA and the scanning voltage VS have been applied at the same time, the voltage between electrodes exceeds a discharge starting voltage for generating a discharge. This type of discharging is regarded as address discharging.
In order to stabilize the address discharging, a priming discharging period is usually provided before address discharging, wherein a voltage waveform, as shown in FIG. 3, is furnished to each electrode, and all cells are turned off after they are illuminated for a moment simultaneously to furnish a predetermined charge (hereinafter, referred to as a wall charge) on the dielectric layer covering the electrode, for initializing all of the cells. In a cell in which a discharge has occurred, charges are accumulated on the dielectric layer covering the electrode, and so as a discharge can be generated again at a lower voltage than the discharge starting voltage if initiated within a predetermined period thereafter. Such a driving method is called a "Memory driving method".
A time sharing drive method (hereinafter referred to as a sub-field method), using this memory driving method, will be described. The sub-field method operates to divide one field into a plurality of sub-fields on which weighting has been effected in accordance with differences in luminous brightness and to select any sub-field for each pixel in response to the magnitude of the applied signal to thereby produce a multi-tone display.
A drive sequence based on the time sharing drive method (sub-field method), as seen in FIG. 4, represents an example of a case where sixteen tones are displayed by means of four sub-fields SF1 to SF4. The scanning period (called an address period as well) 61 represents a period in which a luminescent cell is selected for the first sub-field, and the sustaining period 62 represents a period in which the selected cell emits light in response to a discharging generated between electrodes 27 and 28. The scanning period 61 includes the priming discharge period 63 and a period required to actually determine the address and select the luminescent cell. The priming discharging period 63 is a period required to initialize all the cells by first furnishing a predetermined wall charge on the electrodes on the entire screen.
The sustaining periods for sub-fields SF1 to SF4 are obtained by effecting weighting according to the brightness ratio of 8:4:2:1, and if these sub-fields are arbitrarily selected in response to the level of a video signal, a multi-tone display of the fourth power of 2=16 tones becomes possible. If the number of display gradations need to be increased, the number of sub-fields can be increased, so that, if the number of sub-fields is, for example, 8,256 tones can be displayed. The brightness level of each sub-field is controlled by the number of pulses.
The time sharing drive method, which is characterized by the fact that the scanning period 61 and the sustaining period 62 are thus completely separated from each other and a driving pulse common to all the screens is furnished concerning the sustaining period, is called an "Address display period separated driving method". As regards devices using a time sharing drive method of this type, refer to, for example, SHINGAKU GIHOU EID92-86 (1993-01, pp. 7-11).